Wave amplification



D86. 3, 1940. 5 BLACK 2,223,506

WAVE AMPLIFICATION Original Filed April 22, 1932 3 Sheets-Sheet 1 FIG.

244 24/ 27:9 240 242 243 eopu vg g 252 I J 276 (N A 364 A 257 238 275 L 27/ 278 273 257 IL 263 TI 266 w 2% 255 26 25 .110 no as 100.0: .1 MAL 1o 20 so 4av Supp/Id! POLAR PLOT OFJt Q FOR THE FE EDBACK AMPLIFIER 0F FIG.

O lNl/ENTOR H. .5. BLACK A 7' 7' ORA/E Dec. 3, 1940.

GAIN-D8 2' AND J HARMON/CS, D8 BELOW FUNDAMENTAL H. 5. BLACK 2,223,506

WAVE AMPLIFI CATION Original Filed April 22, 1932 3 Sheets-Sheet 2 FIG. 3

GAIN *WEOUENCY CHARACTER/S TIC AS FUNCTION OF DEEE'NERATIVE FEEDBACK nooaosma/urr AT 10 KC.

OPERATING RANGE I 60- Q rzsoucx No FEEDBACK 50- 95 Fannie/r a: Q FEEDBACK 50.0 o Q 93 (D3 AT/DKC.

0 no 2 4 saw 2 4 65:0 2 4 6810 I00 no 120 I I I FREQUENCY, crcu's PLATE BATTERY voLnet FEEDBACK I00 60- INPUT msauew N0 rezone/r ATZ5/(C.

00- GAIN vs. OUTPUT AT /o/rc.

i 50- N0 'FEEDBA CK FEEDBACK mpur msoumcr AT 51m.

o T o r y o 20 40 so IO |5 2o 30 40 so 6010 so OUTPUT FUNDAMENTAL, M/LS aurpur cup/@5111, M/Ls INVENTOR ATTORNEY Dec. 3, 1940. H. s. BLACK WAVE AMPLIFICATION Origifial Filed April 22, 1932 3 Sheets-Sheet 3 w "III I III I.III4 15 I//+ m a ws 4 m a c M M M 6 7 '3 2 a .I. F o A PW x \I c 4 M. Km R C M 4 M M C 5 I I w \m a. I 2 I I\\ z m o. A o m m CHANGE IN GAIN OF AN AMPLIFIER WITHOUT FEED8ACK,IN DB PLATE VOLTAGE NORMAL OPERATING VOLTAGE 250i 1 VOLT wve/vmq H. 5. BLACK ATTORNEY Patented Dec. 3, 1940 UNITED STATES WAVE AIVIIPLIFICATION Harold S. Black, Elmhurst, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application April 22, 1932, Serial No.

606,871. Divided and this application Novem- 'ber 10, 1937, Serial No. 173,749

7 Claims.

The present invention relates to wave amplifying systems employing negative feedback for improvingthe operation of the system; for example, for increasing linearity or improving a stability.

This is a division of my prior application Serial No. 606,871, filed April 22, 1932, Patent No. 2,102,671, issued December 21, 1937.

The generalobject of the invention is to provide, in novel manner, negative feedback in requisite amount to secure the desired degree of improvement in; transmission characteristic; and more especially in a broad band amplifier emplaying a number of stages in tandem.

The basic theory of operation of amplifiers provided with negative feedback and the design procedure therefor are quite fullyset forth in the parent patent to which reference may be made. It is there pointed out that the use of a large amount of negative feedback is capable of reducing the'modulation or distortion in an amplifier substantially decibel for decibel and likewise improves the stability of gain and other properties substantially in proportion to the gain reduction resulting from the negative feedback. In the same way that the distortion is improved and in like degree: noise and all other variations that arise in the amplifier itself are reduced by the negative feedback.

. The inventionis capable of broad application in the field of wave amplification generally and the basic principle of the invention is not confined to the amplification of electrical waves.

The nature and objects of the invention will be more fully understood from the consideration of a specific example. which will be described in detail in the following specification and illustrated in the accompanying drawings.

Fig. 1 is a circuit diagram of a negative feedback amplifier suitable, for example, for amplifying waves of a frequency range extending from 4 kilocycles to 40 kilocycles in a non-loaded cable used for multiplex carrier telephony; and

Figs. 2 to 8 show characteristics of the amplifier of Fig.1.

Fig. 1 shows the circuit of a negative feedback amplifier 240 comprising vacuum tubes 24l, 242 and 243 in cascade .connection, for amplifying wavesof a-frequency range extending from 4 kilocycles to 40 kilocycles received over incoming line or circuit 238 and transmitting the amplified waves to-outgoingline or. circuit 239. The circuits 238 and 239 may be, for example, sections of a non-loaded multiplex carrier telephone cable circuit, the amplifier amplifying simultaneously a number of carrier telephone and/or carrier telegraph messages of channels extending over the 4 kilocycles to 40 kilocycles range.

The incoming circuit comprises an input transformer 244 and is connected to the input side of the amplifier through a bridge circuit 245. The four ratio arms of the bridge comprise the four resistances 246, 241, 248 and 249, respectively, and in the arm containing resistance 246 is also an adjustable phase correcting condenser 246 which balances the effective input capacity of tube 24] and reduces the phase shifts around the feedback loop to values favorable for avoiding danger of the amplifier tending to be self-oscillatory at high frequencies. The circuit 238 is connected across two of the bridge arms in series and forms one diagonal of the bridge. The input circuit of the amplifier is connected across the arm 249.

The output circuit of the amplifier is connected to the outgoing or load circuit 239 through a bridge circuit 250 and a stopping condenser 25l,

the stopping condenser having negligibly low reactance for the waves to be amplified. An output transformer 252 is included in the outgoing circuit. The ratio arms of the bridge are resistances R0, KRo, KR and R, K being a constant and R0 being the space path resistance of tube 243. A stopping condenser 259 is included in the arm that contains resistance R. The circuit 239 is the output diagonal of the bridge.

Across the resistances KR and KRo in series is connected the input end of a feedback path for the amplifier, comprising conductor or feedback lead 253 and ground, the output end of this path being connected across the arms 24! and 248 of bridge 245. Thus, the feedback path is a diagonal (the feedback diagonal) of output bridge 250, and is also a diagonal of the input bridge 245, as in the case of Fig. 5 of the parent patent.

Tubes 24! and 242 are heater type screen grid tubes of high amplification factor (Western Electric Company type 245-A tubes). Tube 243 is a coplanar grid tube of the general type disclosed by H. A. Pidgeon and J. O. McNally in their copending application Serial No. 368,647, filed June 5,1929, or in their copending application Serial No. 542,252, filed June 5, 1931, (which have now patented as No. 1,923,686, August22, 1933, and No. 1,920,274, August 1, 1933, respectively),

or in their paper published in the Proceedings ofthe Institute of Radio Engineers, vol. 18, pages 226 to 293, February, 1930. Such a tube has two grids, each active elementary area on either grid being close to a. corresponding active area on the other grid and beingat substantially the same location as that corresponding area with respect to the cathode and the anode or plate. For example, each grid may have its active area in the same surface, for instance the same plane or cylindrical surface, as the active surface of the lother grid. As brought out in the above-menhigh value of control grid negative biasing potential and a control grid input voltage wave of large amplitude (i. e., a large grid swing).and a high positive biasing voltage on thespace chargerespective tubes' through those resistors.

grid. Tube type 28l-A tube operated in that way,

263 is a Western Electric Company with grid 2541 serving as a control grid receiving the signal: voltagetransmitted from tube 242; and with grid; 2% serving as a space charge grid. Grid is maintained at negative potential by a negative biasing volt-age applied from a 70-volt battery or source Ew through resistance 251. Grid 255 is maintained positive by a positive biasing voltage applied from a '70-volt. battery or source 258 1 volt plate battery or source filament heating battery or source 26i sending across which is shunted a by-pass condenser 258.

'I ubesfWtl, 232 and 2 33 have a common 130- 26B and 24-V0lt heating currents through, the filaments of the threetubes in series, the positive pole of the filament heating battery being shown connected to thenegative poleof the plate battery.

Plate-current for tube 243 passes from battery 26!! through choke coil 2G2 and the primary winding of output transformer 252 to the plate of the tube. This direct current is prevented from reaching resistance R by the stopping condenser 259 and is prevented frem reaching resistance KRO by condenser 25| which is a by-pass condenser for waves of the frequency to be amplified and which cooperates with the choke coil 282 and a condenser 2'40 to prevent voltage fluecircuit from reaching the alternating current plate amplifier tuations in the battery plat and toprevent the voltage. from-causingfeedback in the through. the common battery circuit. 7 "Plate current for tube 2 passes from battery 260 to the plate through a resistance 263 and an interstage. coupling impedanceor choke coil 265. The resistance 2S3 and a condenser 265 in con-u junction with the condenser 210, form a frequency selective circuit for preventing voltage fluctuations in the battery circuit from reaching the-plate and for preventing the waves inthe alternating current output circuit of the tube from passing to portions of the plate battery circuit common to, a plurality oftubes of theamplifier. The condenser 265 is a by-pass condenser for-wayesof the frequency to be amplified.

Plate current for tube 242 passes from battery 2% through a resistance 25.6 anda choke coil 261 to the plate of the tube. Elements 266 -t0 2% function in connection with tube 242 in the manner in which elements 263 to 2$5iunction in con nection with .tube24 l .Battery 25!]. supplies steady positive biasing potential for the screen grid :of .tube 2M through a 'frequencylselective network comprising :a series resistancearm 2H and shuntcapacity .arms

[212 andZW, and supplies steady positive biasing potential for the screen grid ofitube 2 322 through airequency selective network comprising a series resistance arm 213 and shunt 'capacity'arms 21 4 :and 21.0; These :networks prevent voltage variations; :in; the plate. battery circuit. from reaching the screen grids, and preventwaves in i the screen grid circuits from passing to portions l utilized frequency range.

Negative biasing potentialsv for thecontrol grids of tubes 2M and 242 are obtained from the Voltages .acrossresistors2i5 and 2%, respectively,

that result fromgfio'w of the space currents of the The voltage across resistor 275 reaches the control grid of tube 24-! through the bridge 245 and conductor 225i and the secondary winding of transformer 2%. The voltage across resistor 216 reaches the control grid of tube 242 through grid leakresistor 218." Condensers-219 and 280 in the interstag e coupling =cir cuits ;are. stop1: ingcon'-v densers, of negligibly; low reactance for the ;frequencies of the waves'tozbe transmitted.- Resistances 215 and 216 are notby-passed'for alternating currents and hence iorm a common atef ;circuit' :and. grid impedance between 'the circuit of their; respecti tubes and produce negative feedback stabilizi egain introduced by these two tubes 245la11d242. v

From Fig. zgwhichis apolar plot ofufi for {the amplifier of Fig. 1,-,the.va=1ues of, pl for various frequencies. can be seeing; Eorgexa'mple; for? -fre-, quencies in the. neighborhopd; ch10 kilocycles M51 is approximately 10,0.. i

. Fig. 3 shows.the-gainsfrequency characteristic of this ampliflerpfFig; 1 without feedbackpand with .two different adjustments of the ,circuit as regards :amounts. of negative v ;feedb"ack, the curve labeled Feedback J. corresponding :to one of these two. adjustments and the curveiilabeled Feedback. 2:

corresponding. to the other-10f 'th'esetwlo. adjustments. The three curvesishowhowthegain var.-

ies with frequency as thezamount.oi-feedback'is changed,-. and. make: it? readilyrapp arent: that the gain .variation :withf frequency; is. far. .less in; the feedback rcondition. .1;The; gain includes; that due to the. input and output transformers. Theiline' labeled Operating range in.Fig.;3;ext.ends.from 4 kilocycles .to 40 kilocyclesitolindicate'.thatthe amplifier is designed for operation'lover :thatfrequencyrangep. 1 j Fig. 4 shows how negative feedback .afEectsvariations .ofthe/gain of qthi-sziamplifierwofFig. 1

caused by variationof plate batteryvolta'ge. The

curves of thisfigure. werelltaken at -IO kilo'cycles. the upper curvez'with'no :feedback and the lower one with negative'feedbaclcx:They'show'thatithe gain variations are reducecllbyv the negativ feedbackby an amount corresponding to the gain reduction at the :frequencyo'f measurement: 1 This is 48 dbpat 10 :kilocycles a's s'hown in the'curlve labeled Feedback I in Fig.'3.i= 1 T Fig; 5 gives curves obtained with :this amplifier of Fig. 1, showing the efiect of negative ifeedback upon second harmonic-and vthi-rdha'rm'onic,

for the same outputs of-fundamental (-in rnilli ampere's or wattsi with 'feedback as with'out feedback The curves taken without lfeedback are-labeled fNo feedbackib -The curves flabeled 70.

Feedback '1 are taken withfrtheIsameadjustment of the circuit as was. used fdlifthe 1curve labeled Feedback-..| .in.Fig.-. The frequency. ofqithe fundamental wave used. taking the curves. of the. second: harmonics was. '7 .5ikilocycles; arid-the fundamentalfrequency used in taking the. third harmonic curves was kilocycles. It can be seen that upt'o about'35 milliamperes output of fund,- amental, the negative feedbackreduces the ratio of harmonics to fundamental by approximately the amount of the gainreduction. (The gain reductions at 7.5 kilocycles and at 5 kilocycles can be read from" the curves labeled No feedback and Feedback I in Fig. 3.)

Fig. 6 gives the gain-load characteristics of the amplifier of Fig. 1 with and without feedback. The curve labeled Feedback I was taken at kilocycles and with the same circuit adjustment as the curves labeled Feedback I in Fig. 5. The curves of Fig. 6 show that the negative feedback greatly reduces the change of gain with level. The output current is given in milliamperes into a fixed resistance as usual in such curves.

Fig. 7 shows curves representative of the gain stability of a single amplifier such as that of Fig. 1 as determined by an average of the variations in 69 such amplifiers connected in tandem (in a non-loaded carrier telephone cable circuit approximately 1700 miles long). In comparison of this figure with Fig. 2, which gives a plot of p for a large range of frequencies (for the amplifier of Fig. 1 which is representative of each of the 69 amplifiers in tandem) and the stability boundaries in Figs. 2, 3 and 4 of the parent patent, it will be noted that there are four frequencies for which p] and 1: satisfy boundary C and result in perfect stability. Frequencies between approximately 200 cycles and 1300 cycles should all decrease in gain as the a of the amplifier is increased; While frequencies between approximately 1300 cycles and 30,000 cycles should increase in gain with an increase in and likewise frequencies between approximately 30,000 cycles and 200,000 cycles should decrease with any increase in a. Fig. 7 demonstrates that the experimental evidence supports the theoretical deductions.

To illustrate how the amplifier of Fig. 1 cperates on each of the other boundaries, D, E, G and I, these boundary lines have been drawn in or indicated on the polar plot of Fig. 2. From reference to that figure it is seen that the amplifier operates on boundary-D at a frequency of approximately 200 cycles; on boundary E at two frequencies, respectively; just below 2 kilocycles and just above 30 kilocycles; on boundary G at approximately 12 kilocycles; and on boundary I at four frequencies, respectively, about 280 cycles, approximately 1900 cycles, approximate- 1y 31 kilocycles and approximately 180 kilocycles. Boundaries B and H are also indicated on this same figure but the measurements were not carried far enough to determine the relationship of the operating characteristic to either of these boundaries. It will be apparent from the curve, however, that with the proper circuit constants the amplifier could be made to operate at some one or more frequencies desired on either of these latter boundaries.

Fig. 8 shows curves of stability of gain with respect to varying plate voltage plotted as abscissae. Within the limits indicated on this figure, it is found that the decibel change in gain without feedback is substantially proportional to change in anode voltage, so that the curves on this plot should have substantially the same shape as those in Fig. 7. It will be seen that this is the case, considering that a more open scale is used for ordinates in Fig. 8. The plots iniFig. 8 are also from 'datameasured on'69 amplifiers in tandem, as in the. case of Fig.7, and show how much the gain variesvwith feedback for a .given change in plate.voltage which as stated is approximately proportional, to the changein gain without feedback. A comparison of-.this:Fig. 8 with the polar plot of. Fig. 2 and the. conditions for boundary 0 in Figs..2, .3 and 4 of the parent patent shows the variations in stability with various values of ,ufl. From Fig. 2 it can be seen that there are four frequencies for which the stability is perfect. Fig. 7 shows two of the cross-over points approximately 1300 cycles and 25,000 cycles.

What is claimed is:

1. A three stage amplifier comprising two voltage amplifier stages followed by a power stage, a negative feedback path from the output of the last to the input of the first stage feeding back a portion of the output waves in sufiicient amount to improve the linearity of said amplifier substantially in proportion to the extent of the gain reduction due to the feedback, and a local negative feedback path for each of said voltage amplifier stages for stabilizing the gain introduced by these two stages.

2. An amplifier according to claim 1 for amplifying waves of a broad band of frequencies in l which said local feedback path in each instance comprises an impedance from cathode to ground common to the input and output circuits of the respective stage and eifective to reduce the gain of the respective stage substantially at all of the frequencies in said band.

3. In an amplifier subject to variations in gain from variations in circuit parameters and tending to distort the wave form of the waves being amplified, one gain reducing negative feedback path for stabilizing the gain substantially in proportion to the amount of reduction in gain produced by said feedback and another negative feedback path for reducing the distortion in said amplifier.

4. In an amplifier circuit, a space discharge device having a cathode, a grid and an anode, an output circuit therefor, a source of anode voltage having its negative terminal connected to ground, a signal input circuit connected between the grid and ground, means making the cathode potential above ground for both direct current and signal voltages, whereby the grid is supplied with degenerative signal and negative biasing voltages, and a negative feedback connection from the output side to the input side of said amplifier feeding back a portion of the output waves in sufficient amount to improve the linearity of said amplifier substantially in proportion to the extent of the gain reduction due to the feedback.

5. In an amplifier circuit comprising a plu.

7 rent, both portions being subject to unstable operation, one gain reducing feedback around or g the outputdof the .power stage to theinput of. the voltage amplifying \portion' for t improving. the linearity and stabilizing the gain of said amplie fier and a separate gain-reducing feedback path around the voltage amplifying portion for further stabilizing the-gain of that portion of the amplifier HAROLD S. BLACK. 

